TurboJ
As Blackwidow puts it, it is something ops or crewing can reasonably ask a Commander to 'consider', but that is as far as it goes. It is a commanders responsibility to consider his whole crew's fitness to continue a flight into the discretionary period and that is something ops or crewing have no part in.
That said, it is crutially important that if a commander is acting on information from Ops regarding the operational situation and if that information influences his decision to exercise discretion, then Ops must ensure it is, as best they can, operationally realistic. For example, if your FDP/Discretionary period calculations are based upon a turnaround of 40minutes at an airport you best be sure you're going to achiveve that.